Polymeric and copolymeric materials possessing ferroelectric characteristics have been known in the art for a number of years. Ferroelectricity refers to the generation of an electrical signal in response to a change in a physical stimulus. Ferroelectric characteristics can be broken down into one of two general categories: piezoelectric and pyroelectric materials. Pyroelectric materials generate an electrical signal in response to a change in temperature sensed by the material. Piezoelectric materials generate an electrical signal in response to a change in localized pressure. Each of these effects result from changes in polarization that require a redistribution of surface charges in the material.
Ferroelectric materials exhibit a ferroelectric to paraelectric transition known as the Curie transition temperature (Tc). When subjecting a ferroelectric material to temperatures above the Tc, dipoles of the material become mobile. By applying an electrical field to the material while it is above the Tc, polarized regions of the material will become preferentially oriented. By maintaining the field applied to the ferroelectric material as it is cooled below the Tc, the orientation polarization remains. The remanent polarization disappears if the sample again is heated to or above Tc. The dipoles once again become mobile and can reorient themselves in random order in the absence of an applied electric field. Thus, Tc is the temperature at which polarization, and hence ferroelectric response, is lost. Devices relying on ferroelectric responses generated by polarized ferroelectric polymers can therefore only be used for applications occurring below their Curie transition temperature. Once Tc has been exceeded, the material must be "reset" by applying a polarizing field at temperatures above Tc.
One ferroelectric polymer which has been the subject of substantial investigation is polyvinylidene fluoride ("PVDF"). PVDF has been widely studied due to the variety of its crystalline phases. At least five crystalline phases have been reported for PVDF, three of which are polar ferroelectric, crystalline forms. Accordingly, PVDF and its copolymers exhibit relatively good piezoelectric and pyroelectric response.
Numerous devices have been fabricated with polymeric materials, such as PVDF, taking advantage of the piezoelectric and/or pyroelectric effect thereof. For example, U.S. Pat. No. 4,666,198 to Heiserman describes a piezoelectric polymer microgripper device. Similarly, U.S. Pat. No. 5,089,741 to Park, et al describes a piezofilm impact detector with pyro-effect elimination. Finally, a publication to Brown, from ATOCHEM Sensor Ltd., dated Dec. 14, 1990 describes a thermal detector taking advantage of the piezoelectric and pyroelectric characteristics of PVDF copolymers.
However, each of the above cited references makes use of the piezoelectric and pyroelectric characteristics of the materials. That is, the devices rely upon the electrical current generated by the material in response to a change in physical stimuli. Requiring the device into which the PVDF polymeric or copolymeric material is incorporated to have the ability to read the generated electrical current substantially increases the complexity of the device. This is attributable in part to the fact that the piezoelectric and pyroelectric devices are active in that they are generating electrical current.
Accordingly, there exists a need for a thermal sensing device which takes advantage of the inherent characteristics of PVDF polymers and copolymers, without regard to the piezoelectric and pyroelectric characteristics thereof. The device should be fairly simple in that it should not require additional circuit elements to be incorporated into the circuit into which it is disposed, and should be reliable so as to be readily usable over many cycles, and in a wide range of temperatures and frequencies.